Magnatec just seems to make them run like a bag of spanners from cold for some reason! Does so especially badly on valvers and williams and I think its similar for 172s etc.
Not all renault places hold the fully synth Excellium - best bet is to ring and see if they have any in stock. I think book price is about £35 for 5 litres but hopefully youll be able to get it for less.
Oilmans Silkolene Pro S(?) 5W40 is apparently the mutts nutts though so could also be worth speaking to him, but you might end up paying for postage if youre not going to the shows to pick it up.
Not sure on the oil capacity of a 172 though - 5 litres *should* be enough but check your handbook or wait until someone more knowledgable on here answers
